1. Field
The present invention relates to processes for producing a catalyst-layer-supporting substrate and to a catalyst-layer-supporting substrate, a membrane electrode assembly, and a fuel cell.
2. Description of the Related Art
Polymer fuel cells, for example, methanol type polymer fuel cells in which a methanol solution is used as a fuel, can work at low temperatures and can be reduced in size and weight. Because of this, polymer fuel cells are recently being enthusiastically investigated as power sources for mobile appliances, etc. However, the electrodes of such fuel cells mainly employ noble-metal catalysts and, hence, the fuel cells are highly costly and have not yet reached such a level that the fuel cells can spread widely. There is a desire for a technique which brings about high fuel cell performance with a small amount of a noble-metal catalyst.
For producing electrodes for conventional fuel cells, a method is generally employed in which a noble-metal catalyst, a proton conductor, a solvent, etc. are mixed together to produce a slurry and this slurry is applied to a substrate. This method, however, has a problem that the noble-metal loss in the process is as large as about 30%.
Methods of electrode production by sputtering or vapor deposition are being investigated as techniques reduced in noble-metal loss in the process.
For example, a technique in which a catalyst metal such as, e.g., a noble metal is sputtered on a substrate and a layer of particulate carbon is then formed thereon has been reported in WO 2002/073722. However, use of this technique in producing a catalyst layer has a problem that fine catalyst metal particles aggregate to form large particles, resulting in an insufficient efficiency of catalyst metal utilization.
On the other hand, a technique has been reported in which an easy-to-dissolve metal is used as a pore-forming metal to form an alloy or mixture thereof with a catalyst metal by sputtering or vapor deposition and a porous structure is thereafter formed in the resultant catalyst layer by a pore formation process (U.S. Pat. No. 4,126,934 and U.S. Patent Publication No. 2006/0189132). Furthermore, a technique has been reported which includes sputtering a catalyst metal, subsequently forming a layer of a pore-forming metal by sputtering, and then forming a porous structure in the catalyst layer by a pore formation process (JP-T 2007-507328). However, all these techniques are insufficient in the efficiency of catalyst utilization. A further improvement is desired.